Real-time lighting effects like soft shadows and subsurface scattering can be obtained with graphics rendering techniques that involve precomputed radiance transfer (PRT). Images generated using these techniques exhibit a compelling realism lacking from traditional computer graphics with its emphasis on hard shadows and opaque surfaces. However, PRT techniques have been limited to static objects or highly constrained, precomputed sequences.
PRT methods for static objects can be categorized as low-frequency (such as described by Sloan, P., Kautz, J., and Snyder, J., “Precomputed Radiance Transfer For Real-Time Rendering In Dynamic, Low-Frequency Lighting Environments,” Proc. of SIGGRAPH '02, 527-536; and Sloan, P., Hall, J., Hart, J., and Snyder, J., “Clustered Principal Components For Precomputed Radiance Transfer,” Proc. of SIGGRAPH '03, 382-391) and all-frequency (such as described by Ng, R., Ramamoorthi, R., and Hanrahan, P., “All-Frequency Shadows Using Non-Linear Wavelet Lighting Approximation,” Proc. of SIGGRAPH '03, 376-381; Liu, X., Sloan, P., Shum, H., and Snyder, J., “All-Frequency Precomputed Radiance Transfer For Glossy Objects,” Proc. of 2004 Eurographics Symposium on Rendering; and Wang, R., Tran, J., and Luebke, D., “All-Frequency Relighting Of Non-Diffuse Objects Using Separable BRDF Approximation,” Proc. of 2004 Eurographics Symposium on Rendering), according to the lighting and shading frequencies handled. Though more general, all-frequency PRT is problematic because it tabulates much more data per point to represent transfer from higher-dimensional lighting vectors, and many more points per object to represent higher frequency shading variation over that object. The resulting data sets are enormous, and remain unwieldy even after sophisticated compression techniques are applied.
PRT has also been applied to deformable models (described by James, D., And Fatahalian, K., “Precomputing Interactive Dynamic Deformable Scenes,” Proc. of SIGGRAPH '03, 879-887) but the preprocessing was essentially done for each pose of the object in a low-dimensional control space: the space of elastic deformations caused by an impulse at an arbitrary instant (i.e., a “poke”). Models can only be poked, they can't be bent or twisted, walk, or flap their wings. This did not permit capturing local effects for models where the deformations are not known in advance, and so can have arbitrarily many degrees of freedom.
Bi-scale radiance transfer (described by Sloan, P., Liu, X., Shum, H., And Snyder, J., “Bi-Scale Radiance Transfer,” Proc. of SIGGRAPH '03, 370-375) and environmental lighting with BTFs (described by Muller, G., Meseth, J., And Klein, R., “Fast Environmental Lighting For Local-PCA Encoded BTFs,” Proc. of Computer Graphics International, 198-205) capture both local and distant transfer effects on static objects. The distant level, represented as transfer matrices, is required not only to obtain distant effects but also to rotate the lighting vector to the local frame. If the object deforms, this rotation is impractical to tabulate and so must be performed dynamically at each surface point. These methods also capture view dependent parallax effects using 4D textures.